Power Supply Circuit and Power Conversion Device

ABSTRACT

A power conversion device includes an inverter circuit converting DC power into AC power and including switching devices constituting upper and lower arms, a control circuit controlling the switching devices, a drive circuit driving the switching devices by a signal from the control circuit, and an insulated power supply circuit supplying power to the drive circuit. The control circuit controls a power supply voltage to be outputted from the power supply circuit to the drive circuit. The drive circuit drives the switching devices and based on a carrier frequency and the power supply voltage. The power supply circuit includes a feedback output circuit through which the voltage outputted to the drive circuit is outputted to a power supply control IC. The feedback output circuit includes a dummy load circuit which controls the voltage to be outputted to the power supply control IC based on a change of the carrier frequency.

BACKGROUND OF THE INVENTION

The present invention relates to a power supply circuit and a powerconversion device for a hybrid car or an electric car.

With increase in the switching speed of a switching device formed as aconstituent member of an inverter, the switching loss is reduced, butsurge is apt to occur in the collector-emitter voltage of the switchingdevice. In this case, there is a problem that the switching device maybe damaged when the voltage is beyond its rated voltage. On the otherhand, when the switching speed is slow, surge hardly occurs, but theswitching loss of the switching device is large to cause deteriorationof the energy efficiency. In addition, when the switching speed is slow,the junction temperature is apt to increase so that the switching devicemay be damaged when the temperature is beyond its rated temperature.

A gate drive circuit needs to be designed to optimize these trade-offs.It is desirable that a gate voltage does not fluctuate. In order to keepthe gate voltage constant, it is necessary to make the output voltage ofa power supply circuit not to fluctuate.

In a background-art power supply circuit, with increase in the carrierfrequency (switching frequency) of a gate drive circuit, a current withwhich the capacity in the gate of a switching device ischarged/discharged increases to increase power consumption in the gate.On the other hand, a feedback output circuit does not respond to thecarrier frequency fc as the gate drive circuit does. As a result, inspite of the increase of power consumption in the power supply circuit,there is no change in an output voltage 601 of the feedback outputcircuit as represented in FIG. 6. Thus, the electric power supplied tothe gate drive circuit is kept constant in spite of the fluctuation ofthe carrier frequency fc. Even if the power supplied to the gate drivecircuit is constant, the power consumption of the gate drive circuitincreases. Thus, the power supply voltage drops down as represented inthe power supply voltage Vcc of a secondary output circuit in FIG. 6.

As a result, the gate voltage of the switching device is lower than thatin the optimized design condition. Thus, there arises a problem that theenergy efficiency deteriorates due to the increase in switching loss.

JP-A-2005-341695 discloses an invention of a power supply circuit inwhich a dummy load circuit provided in an output circuit is turned ON inresponse to a decrease in a load of the output circuit so as to suppressan increase in an output voltage by increasing the load.

The power supply circuit disclosed in JP-A-2005-341695 indeed takesmeasures to solve the problem that the output voltage of the outputcircuit increases when the load of the output circuit decreases,however, there is no consideration for the problem that the outputvoltage of the output circuit decreases when the load of the outputcircuit increases.

SUMMARY OF THE INVENTION

In consideration of the aforementioned problem, an object of theinvention is to provide a power conversion device which can suppress thedecrease of output voltage even when the carrier frequency increases.

The power conversion device according to the invention includes aninverter circuit which converts DC power into AC power, and furtherincludes a plurality of switching devices constituting upper and lowerarms, a control circuit which controls the switching devices, a drivecircuit which drives the switching devices based on a signal from thecontrol circuit, and an insulated power supply circuit which suppliespower to the drive circuit, wherein the control circuit controls a powersupply voltage to be outputted from the power supply circuit to thedrive circuit, the drive circuit drives the switching devices based on acarrier frequency and the power supply voltage, the power supply circuitincludes a feedback output circuit through which the voltage outputtedto the drive circuit is outputted to a power supply control IC, and thefeedback output circuit includes a dummy load circuit which controls thevoltage to be outputted to the power supply control IC based on a changeof the carrier frequency.

It is possible to provide a power conversion device which can suppress adecrease of the output voltage to the gate even if the carrier frequencyincreases.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a buffer circuit according to theinvention;

FIG. 2 is a circuit diagram of a power supply circuit according to afirst embodiment of the invention;

FIG. 3 is a diagram representing an example of an electric circuitconfiguration of an inverter for driving a three-phase motor;

FIG. 4 is a circuit diagram of a dummy load circuit according to thefirst embodiment of the invention;

FIG. 5 is a circuit diagram representing a configuration example of agate drive circuit;

FIG. 6 is a graph representing a carrier frequency dependency of a powersupply voltage in a power supply circuit according to an example of theprior art;

FIG. 7 is a graph representing a carrier frequency dependency of a powersupply voltage in a power supply circuit according to the invention;

FIG. 8 is a circuit diagram of a power supply circuit according to asecond embodiment of the invention;

FIG. 9 is a circuit diagram of a dummy load circuit according to thesecond embodiment of the invention;

FIG. 10 is a circuit diagram of a power supply circuit according to athird embodiment of the invention;

FIG. 11 is a circuit diagram of a dummy load circuit according to thethird embodiment of the invention;

FIG. 12 is a circuit diagram of a power supply circuit according to afourth embodiment of the invention; and

FIG. 13 is a circuit diagram of a dummy load circuit according to thefourth embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In addition to the contents of the aforementioned “SUMMARY OF THEINVENTION”, the following embodiments can attain desired objects forcommercialization as products and obtain preferable effects oncommercialization as products. Some of them will be described below, andspecific solutions to the problem and specific effects will be describedin “DETAILED DESCRIPTION OF THE EMBODIMENTS”.

First Embodiment

A first embodiment of the invention will be described with reference toFIG. 2. FIG. 2 represents a power supply circuit for gate drivecircuits. The power supply circuit is a flyback type circuit. The powersupply circuit has a transformer 103 for insulating outputs. Thetransformer 103 has one primary coil 222 and seven secondary coils 220and 221. One of the secondary coils is a dummy coil 220 for feeding backa power supply voltage, and the other six 221 provide power to gatedrive circuits 106. A power supply control IC 101 and a transformerdriving MOSFET 102 are located on the primary side of the transformer.The primary coil is connected to a battery 109 outside an inverterthrough a motor control board. The power supply control IC 101 outputs aPWM signal to the gate of the MOSFET 102 so as to switch a primarycurrent of the transformer 103. On that occasion, the primary current ofthe transformer 103 can be changed in accordance with the duty ratio ofthe PWM signal so as to change the power to be transmitted from thebattery 109 to the secondary side. The reference potential of thebattery 109 is set at a body of a vehicle. The reference potential ofthe battery 109 is different from the reference potential of ahigh-voltage DC power supply 306 which supplies power to a motor. Eachsecondary coil includes a rectifier diode 104 and a capacitor 105. Anoutput appears between the terminals of the capacitor. To the output,gate drive circuits, 106UP, 106UN, 106VP, 106VN, 106WP, 106WN(hereinafter referred to as gate drive circuit 106) corresponding toeach of the gate of the secondary coil are connected as a load. A totalof six circuits like this correspond to upper and lower arms of threephases. Here, a power supply circuit 190 shown in FIG. 3 uses thebattery 109 as a power supply, and the reference potential of the powersupply circuit 190 is set at the vehicle body. It is therefore necessaryto insulate the power supply circuit 190 from the reference potential ofeach secondary output circuit. Thus, the voltage of the output circuitcannot be fed back directly. For this reason, a seventh secondary outputcircuit whose reference potential is equal to the reference potential ofthe battery 109 is prepared separately as a feedback output circuit 107.The feedback output circuit 107 also includes a rectifier diode and acapacitor in the same manner as the other secondary output circuits. Inaddition, the secondary coil of the feedback output circuit has the samenumber of turns as any other secondary coil. When the coupling betweenthe coils in the transformer is enough dense, an output voltage equal tothat in any other secondary circuit appears in the feedback outputcircuit 107. The feedback output circuit 107 is provided with a voltagedivider circuit 108 for dividing the output voltage of the feedbackoutput circuit 107 to a predetermined voltage. Here, the voltage dividercircuit feeds an output voltage signal back to the power supply controlIC 101. Then, the power supply control IC 101 detects the output voltageof the feedback output circuit of the power supply circuit and adjuststhe duty ratio of the PWM output signal for switching the transformerdriving MOSFET 102 so as to set the output voltage of the feedbackoutput circuit at a predetermined voltage (15 V). The feedback outputcircuit 107 is provided with a dummy load circuit 201. The dummy loadcircuit 201 has an external input signal terminal 203. The dummy loadcircuit 201 is connected between an output 202 of the feedback outputcircuit and the ground. The external input signal terminal 203 isconnected to a U-phase lower arm gate signal wire 308. Although theexternal input signal terminal 203 is connected to the U-phase lower armgate signal wire 308 by way of example here, the external input signalterminal 203 may be connected to any other gate signal wire.

(Inverter)

An inverter (power conversion device) for driving a motor of a hybridcar has a function by which DC power supplied from a DC power supply canbe converted into AC power to be supplied to an AC electric load such asa rotary electric machine, or a function by which AC power generated bythe rotary electric machine can be converted into DC power to besupplied to the DC power supply. To obtain the conversion function, theinverter includes switching devices such as MOSFETs or IGBTs. Each ofthe switching devices is turned ON/OFF repeatedly to perform the powerconversion from DC power to AC power or from AC power to DC power.

An example of an electric circuit structure of the hybrid car motordriving inverter will be described with reference to FIG. 3. An inverter301 is constituted by a motor control board 302, a gate drive board 303,and a power module 304 with switching devices. The power module 304includes arms in each of which an IGBT 130, 150 and a diode 120, 140 areelectrically connected in parallel. Of the arms, the arm which isdisposed on the positive electrode side of a high-voltage DC powersupply is called an upper arm, and the arm which is disposed on thenegative electrode side of the high-voltage DC power supply is called alower arm. A series circuit 180 is a circuit in which an emitterelectrode 132 of the IGBT forming the upper arm and a collectorelectrode 152 of the IGBT forming the lower arm are electricallyconnected in series. The series circuit 180 has an intermediate outputterminal 155 for outputting power to a motor 310. When the inverterdrives a three-phase motor, the inverter must have three outputs. Thus,three series circuits 180 each having a pair of upper and lower arms ofswitching devices are built in the inverter. In addition, in the gatedrive board 303, gate drive circuits are prepared for the IGBTs 130 andIGBTs 150 of the three pairs of upper and lower arms respectively. Thatis, a total of six gate drive circuits 106 are provided in the gatedrive board 303. A total of six gate signal wires including a powersupply line 309 and a gate signal wire 308 leading to the U-phase lowerarm gate drive circuit 106UN are connected between the motor controlboard 302 and the gate drive board 303.

The following should be noted here. That is, the motor control board 302and each gate drive circuit 106 in the gate drive board 303 havedifferent reference potentials. There are generally two kinds ofexternal power supplies for the hybrid car motor driving inverter. Oneis a normal 12 V power supply for vehicles and the other is thehigh-voltage power supply 306 for driving the motor. Here, the 12 Vpower supply is used for a control circuit, and the reference potentialof the 12 V power supply is set at the body of the vehicle. On the otherhand, the high-voltage power supply 306 for driving the motor is fed tothe IGBTs 130 and 150, and the reference potential of the high-voltagepower supply 306 is not always set at the vehicle body, but theintermediate potential between the positive electrode potential and thenegative electrode potential of the high-voltage power supply may be setat the vehicle body. In this embodiment, the 12 V power supply forvehicle is used for the motor control board 302, and the referencepotential of the 12V power supply is set at the vehicle body. On theother hand, the reference potential of each of the gate drive circuits106UP, 106VP and 106WP for the upper arms is equal to the potential ofthe emitter electrode 132 of the IGBT 130 corresponding to each of thegate drive circuits 106UP, 106VP and 106WP, that is, the potential ofthe intermediate output terminal of the inverter. The referencepotential of each of the gate drive circuits 106UN, 106VN and 106WN forthe lower arms is equal to the potential of the emitter electrode 152 ofthe IGBT 150 corresponding to each of the gate drive circuits 106UN,106VN and 106WN, that is, the negative-electrode-side potential of thehigh-voltage DC power supply 306.

As for the operation of the inverter, first, the motor control board 302transmits PWM gate signals to the six gate drive circuits 106 of thegate drive board 303 in order to switch the IGBTs 130 and 150. Here, dueto the difference in reference potential between the motor control board302 and the gate drive board 303, the signals are transmitted andreceived through insulated signal transmission units such asphoto-couplers. Next, each gate drive circuit 106 applies a voltagebetween the gate terminal and the emitter terminal of the IGBT 130, 150based on the gate signal, so as to switch the IGBT 130, 150. Thus, theIGBTs 130 and 150 apply a current to the motor 310 so that the motor 310is driven.

(Gate Drive Circuit)

FIG. 5 represents a block diagram illustrating one gate drive circuit106 by way of example. A gate driver circuit 501 is chiefly constitutedby a photo-coupler 507, a gate drive IC 509, a buffer circuit 510, apower supply bypass capacitor 560 connected in parallel with the gatedrive IC 509 and the buffer circuit 510, and a capacitor 550 connectedin parallel with the gate terminal and the emitter terminal of the IGBT130, 150.

The gate signal is inputted from the motor control board 302 to the gatedrive IC 509 through the photo-coupler 507. The gate signal inputtedfrom the motor control board 302 has a signal level of 5 V with areference potential set at the vehicle body. On the other hand, the gatedrive circuit 106 has a different reference potential, and the signallevel of the gate drive circuit 106 is 15 V, which is higher than thegate threshold voltage of the IGBT driving a high current. That is, thephoto-coupler 507 also has a role of signal level conversion as well asa role of insulated signal transmission. Based on the signal, the gatedrive IC 509 applies a gate-emitter voltage to the IGBT 130, 150 throughthe buffer circuit 510.

(Buffer Circuit)

The buffer circuit 510 has a configuration in which a resistance 450, anIGBT 460, an IGBT 470 and a resistance 480 are connected in series asrepresented in FIG. 1. A gate resistance 440 is electrically connectedto the gates of the IGBT 460 and the IGBT 470.

(Dummy Load Circuit)

The dummy load circuit 201 will be described with reference to FIG. 4.The circuit is constituted by an external input signal buffer 411, adriver 412, a load capacitance 409, and a load resistance 410. A powersupply terminal and a ground terminal of the dummy load circuit 201 areconnected to the output 202 of the feedback output circuit and theground respectively. A bypass capacitor 413 corresponds to the powersupply bypass capacitor 560 of the gate drive circuit 106. It isdesirable that the bypass capacitor 413 is provided to have the samecapacitance as that of the power supply bypass capacitor 560 placed inthe gate drive circuit 106.

The external input signal buffer 411 converts the signal level of thegate signal from 5 V to 15 V. The external input signal buffer 411serves as a signal inverting circuit, which is constituted by a MOSFET402, a gate resistance 401, and a resistance 403 pulled up to the output202 of the feedback output circuit. The gate signal of 5 V from themotor control board 302 is supplied to the gate terminal of the MOSFET402 through the gate resistance 401. Then, the external input signalbuffer 411 outputs an inversion signal of 15 V to the driver 412 locatedin the next stage.

The driver 412 obtains power supply from the output 202 of the feedbackoutput circuit so as to charge/discharge the load capacitance 409 withthe frequency of the gate signal. The driver 412 serves as anon-inversion buffer circuit, which is constituted by an inputresistance 404, a high-potential-side output resistance 405, a PNPbipolar transistor 406, an NPN bipolar transistor 407, and alow-potential-side output resistance 408.

The load capacitance 409 has a capacitance value CL2 which is equal tothat of the load capacitance of the gate drive circuit, that is, thetotal sum of the gate capacitance of the IGBT 130, 150 of the IGBTmodule 304 and the capacitance existing in the gate drive circuit 106.As a method for deciding the capacitance value specifically, thecapacitance value CL2 may be set as CL2=QG/VG+C0, where QG designatesthe gate accumulated charge of the IGBT 130, 150 and VG designates thegate voltage of the same. Here, C0 designates the total capacitance ofcapacitive loads charged/discharged with the power supply voltage otherthan the output load in the gate drive circuit. In addition, it is morepreferable that the carrier frequency dependency of the power supplyvoltage of the gate drive circuit 106 is actually measured and thecapacitance value CL2 is adjusted experimentally to make the outputvoltage of the feedback output circuit agree with the carrier frequencydependency of the power supply voltage. As another method for decidingthe load capacitance 409 than the aforementioned method, a capacitancesimilar to the load capacitance 550 used in the gate drive circuit 106as represented in FIG. 5 may be used.

The load resistance 410 simulates a DC current load of the gate drivecircuit 106. The load resistance 410 has a resistance value RL whichallows a flow of current equal to a DC component of the current consumedby the gate drive circuit 106. Specifically, it is desirable that theresistance value RL is a resistance value between a gate drive ICpositive electrode connection point 520 and a gate drive IC negativeelectrode connection point 530 in the gate drive IC 509 as representedin FIG. 5. In addition, it is more preferable that the power supplyvoltage between the gate drive IC positive electrode connection point520 and the gate drive IC negative electrode connection point 530 at acarrier frequency of 0 Hz is actually measured and the resistance valueRL is experimentally adjusted to make the output voltage of the feedbackoutput circuit agree with the carrier frequency dependency of the powersupply voltage.

When a real gate drive circuit 106 is used in the external input signalbuffer 411 and the driver 412 of the dummy load circuit 201, thefrequency response of each actual gate drive circuit can be simulated toimprove the accuracy of feedback further.

With this configuration, in the dummy load circuit 201, the driver 412obtains power supply from the output 202 of the feedback output circuitin accordance with the gate signal from the motor control board 302, soas to charge/discharge the same load capacitance as that of the gatedrive circuit 106 at the carrier frequency. The load resistance 410obtains power supply from the output 202 of the feedback output circuitand allows a flow of the same current as the current flowing in the gatedrive IC 509. Thus, a similar load to that of the gate drive circuit 106can be reproduced in the feedback output circuit of the power supplycircuit independently of the carrier frequency.

Based on the aforementioned contents, the load of the feedback outputcircuit may follow an actual load even when the carrier frequencyincreases to increase the load of the output circuit of the power supplycircuit 190. Since the load of the feedback output circuit may followthe actual load, a power supply voltage Vcc 702 of a secondary-sideoutput circuit becomes substantially constant even when the carrierfrequency fc increases, as represented in the graph of FIG. 7. Thus, theoutput voltage drop, that is, the gate voltage drop of each switchingdevice can be suppressed when the carrier frequency increases, so thatit is possible to suppress deterioration of power efficiency in theinverter.

Second Embodiment Power Supply Circuit

A second embodiment of the invention will be described with reference toFIG. 8. This power supply circuit 190 has almost the same configurationas the power supply circuit described in the first embodiment. In thefirst embodiment, power is also supplied to the dummy load circuit sothat the power is also consumed in the dummy load circuit. However,since it works out as long as the dummy load circuit can output afeedback output voltage high enough to prevent the output voltage dropof each gate drive circuit 106, it is therefore desirable that the powerconsumption in the dummy load circuit is suppressed. To this end, thisembodiment differs from the first embodiment as follows. That is, thenumber of turns of a feedback coil 804 of a transformer 803 is reducedto 1/3 of the number of turns of any other secondary coil so as to makethe voltage division ratio of a voltage divider circuit 805 three timesas high as its original one. That is, the output voltage is made 1/3(i.e. from 15 V to 5 V). With the aforementioned configuration, thevoltage outputted to the dummy load circuit 801 can be reduced tosuppress the power consumption in the dummy load circuit. The dummy loadcircuit 801 has an external input signal terminal 808. The dummy loadcircuit 801 is connected between an output 802 of the feedback outputcircuit and the ground. The external input signal terminal 808 isconnected to a U-phase lower arm gate signal wire 308. Although theexternal input signal terminal 808 is connected to the U-phase lower armgate signal wire 308 by way of example here, the external input signalterminal 808 may be connected to any other gate signal wire.

(Dummy Load Circuit)

The dummy load circuit 801 in the second embodiment will be describedwith reference to FIG. 9. The circuit is constituted by a buffer IC 901,a load capacitance 902 and a load resistance 903. A power supplyterminal and a ground terminal of the circuit are connected to theoutput 802 of the feedback output circuit and the ground respectively. Abypass capacitor 904 simulates the power supply bypass capacitor 560connected in parallel with the gate drive circuit 106 so that it isdesirable that the bypass capacitor 904 is provided to have the samecapacitance as that of the power supply bypass capacitor 560 placed inthe gate drive circuit 106.

The buffer IC 901 obtains a 5 V supply from the output 802 of thefeedback output circuit so that the gate signal from the motor controlboard 302 can be inputted directly with a signal level of 5 V so as tocharge/discharge the load capacitance 902 with the frequency of the gatesignal.

The load capacitance 902 is decided in the same manner as in theaforementioned first embodiment.

The load resistance 903 simulates a DC current load of the gate drivecircuit 106. The load resistance 903 has a resistance value RL whichallows a flow of current corresponding to 1/3 of a DC component of thecurrent consumed by the gate drive circuit. Specifically, it isdesirable that the resistance value RL is three times as large as theresistance value between the gate drive IC positive electrode connectionpoint 520 and the gate drive IC negative electrode connection point 530in the gate drive IC 509 as represented in FIG. 5. In addition, it ismore preferable that the power supply voltage between the gate drive ICpositive electrode connection point 520 and the gate drive IC negativeelectrode connection point 530 at a carrier frequency of 0 Hz isactually measured, and the resistance value RL is experimentallyadjusted to make the output voltage of the feedback output circuit agreewith the carrier frequency dependency of the power supply voltage.

With this configuration, in the dummy load circuit 801, the buffer IC901 obtains a power supply from the output 802 of the feedback outputcircuit in accordance with the gate signal from the motor control board302, so as to charge/discharge the same load capacitance as that of thegate drive circuit at the carrier frequency and the voltage which is 1/3as large as the power supply voltage of the gate drive circuit. The loadresistance 903 obtains a power supply from the output 802 of thefeedback output circuit to allow a flow of current corresponding to 1/3of the DC component of the current consumed in the gate drive IC 509.Thus, in the feedback output circuit whose output voltage is scaled downto 1/3 of the power supply voltage of the gate drive circuit 106, a loadwhich is 1/3 as large as the load of the gate drive circuit 106 can bereproduced irrespective of the carrier frequency. In addition, when theoutput voltage is scaled down to 1/3 of the output voltage of the gatedrive circuit 106, the buffer 411 can be dispensed with, and as aresult, the dummy load circuit can be miniaturized. In order to obtainan effect merely to suppress the power consumption, the number of turnsof the feedback coil 804 may be reduced to 1/N as large as the number ofturns of any other secondary coil, so as to make the voltage divisionratio of the voltage divider circuit 805 N times as high as its originalone. Based on the aforementioned contents, the load of the feedbackoutput circuit may follow an actual load even when the carrier frequencyincreases and the load of the output circuit of the power supply circuit190 increases. Since the load of the feedback output circuit may followthe actual load, a power supply voltage Vcc 702 of a secondary-sideoutput circuit becomes substantially constant even when the carrierfrequency fc increases, as represented in the graph of FIG. 7. Thus, theoutput voltage drop, that is, the gate voltage drop of each switchingdevice can be suppressed when the carrier frequency increases, so thatit is possible to suppress deterioration of power efficiency in theinverter. Further, when the output voltage of the feedback outputcircuit is made as high as the gate signal voltage of the motor controlboard 302, the buffer 411 shown in the first embodiment can be dispensedwith. Thus, the dummy load circuit 801 can be simplified so thatminiaturization and low cost can be achieved. In addition, the voltageoutputted to the dummy load circuit 801 is reduced so that the powerconsumed by the dummy load circuit 801 can be reduced to improve theefficiency of the power supply circuit.

Third Embodiment Power Supply Circuit

A third embodiment of the invention will be described with reference toFIG. 10. A power supply circuit in this embodiment is substantially thesame as the power supply circuit described in the first embodiment. Inthe first and second embodiments, a gate signal to be outputted to oneupper arm or one lower arm is supplied to the dummy load circuit.However, when the control system of the inverter 301 is changed to atwo-phase modulation system, there is a fear that feedback cannot beperformed due to use of a signal of one phase which is not operative.Accordingly, in the third embodiment, three signal terminals 1004, 1005and 1006 connected to the motor control board 302 are provided betweenan output node 1002 of a feedback output circuit 1003 and the ground.Thus, signals based on gate signal information, for example, carrierfrequency information can be outputted directly to a dummy load circuit1001 so as to improve the reliability of control.

Assume that fcmax designates the highest one of the carrier frequenciesfc, f0 designates a frequency which is representative of a low bandexpressed by f0=1/4×fcmax, f1 designates a frequency which isrepresentative of a middle band expressed by f1=1/2×fcmax, and f2designates a frequency which is representative of a high band expressedby f2=3/4×fcmax. Of the signals based on the carrier frequencyinformation, the signal to be inputted to the signal terminal 1004becomes a “H” level (5 V) when the carrier frequency fc is higher thanthe frequency f0, while the signal becomes a “L” level (0 V) when thecarrier frequency fc is lower than the frequency f0. The signal to beinputted to the signal terminal 1005 becomes the “H” level (5 V) whenthe carrier frequency fc is higher than the frequency f1, while thesignal becomes the “L” level (0 V) when the carrier frequency fc islower than the frequency f1. The signal to be inputted to the signalterminal 1006 becomes the “H” level (5 V) when the carrier frequency fcis higher than the frequency f2, while the signal becomes the “L” level(0 V) when the carrier frequency fc is lower than the frequency f2.Although the frequencies f0, f1 and f2 representative of the low, middleand high bands are expressed by f0=1/4×fcmax, f1=1/2×fcmax andf2=3/4×fcmax respectively here, the frequencies are not limited to suchsettings, but may be changed in accordance with the sizes of thefrequency bands to be used.

(Dummy Load Circuit)

The dummy load circuit according to the third embodiment of theinvention will be described with reference to FIG. 11. The dummy loadcircuit 1001 is constituted by switch-including DC load circuits 1101,1102 and 1103 and a load resistance 1106. A bypass capacitor 1107corresponds to the power supply bypass capacitor 560 of the gate drivecircuit 106. It is desirable that the bypass capacitor 1107 is providedto have the same capacitance as that of the power supply bypasscapacitor 560 placed in the gate drive circuit 106.

The load resistance 1106 simulates a DC current load of the gate drivecircuit 106. The load resistance 1106 has a resistance value RL whichallows a flow of current equal to a DC component of the current consumedby the gate drive circuit 106. Specifically, it is desirable that theresistance value RL is a resistance value between the gate drive ICpositive electrode connection point 520 and the gate drive IC negativeelectrode connection point 530 in the gate drive IC 509 as representedin FIG. 5. In addition, it is more preferable that the power supplyvoltage between the gate drive IC positive electrode connection point520 and the gate drive IC negative electrode connection point 530 at acarrier frequency of 0 Hz is actually measured, and the resistance valueRL is experimentally adjusted to make the output voltage of the feedbackoutput circuit agree with the carrier frequency dependency of the powersupply voltage. The switch-including DC load circuit 1101 is constitutedby a MOSFET 1104 and a resistance 1105 pulled up to the output node 1002of the feedback output circuit. When the signal terminal 1004 reachesthe “H” level, the MOSFET 1104 is turned ON to allow a flow of currentfrom the output node 1002 of the feedback output circuit to the groundthrough the resistance 1105. The resistance 1105 has a resistance valuehigh enough to allow a flow of current corresponding to the capacitanceload current of the gate drive circuit at the carrier frequency fc=f0.The other switch-including DC load circuits 1102 and 1103 have the sameconfiguration as the switch-including DC load circuit 1101. Resistances1105, 1108 and 1110 have the same resistance value.

The dummy load circuit 1001 allows a flow of current only through theload resistance 1106 when the carrier frequency fc is in a very lowrange of 0<fc<f0. In addition to the load resistance 1106, theswitch-including DC load circuit 1101 is turned ON in a low band rangeof f0<fc<f1 so as to allow a flow of current corresponding to the loadcurrent of the gate drive circuit at the carrier frequency fc=f0. In amiddle band range of f1<fc<f2, the switch-including DC load circuit 1102is turned ON in addition to the load resistance 1106 and theswitch-including DC load circuit 1101 so as to allow a flow of currentcorresponding to the load current of the gate drive circuit at thecarrier frequency fc=f1. Further, in a high band range of f2<fc<fcmax,all of the load resistance 1106 and the switch-including DC loadcircuits 1101, 1102 and 1103 are turned ON to allow a flow of currentcorresponding to the load current of the gate drive circuit at thecarrier frequency fc=f2. In addition, the load capacitance 409 or 902 inthe first or second embodiment is not present in the third embodiment.Thus, the dummy load circuit 1001 can be made smaller in size, leadingto cost reduction of the power supply circuit. Further, the load currentflowing in the dummy load circuit 1001 is outputted not bycharging/discharging with the load capacitance 409 or 902 represented inFIG. 4 or FIG. 9 but only through the resistances 1105, 1108 and 1110.Thus, a ripple noise is prevented from appearing on the output voltageof the feedback output circuit 1003, so that stable control can beperformed.

Although the resistances 1105, 1108 and 1110 are set to have the sameresistance value, they may be set to have different values. Particularlywhen the resistance values of the resistances 1105, 1108 and 1110 areset at the ratio 1:2:4, the current can be varied in eight stages tosupport finer control. Thus, the reliability of feedback can beimproved.

Assume that a switching signal actually outputted to a gate is outputtedto the dummy load circuit as described above. When the duty ratio usedfor the PWM control is changed on this occasion, there is a possibilitythat the pulse width may be too narrow to be sensed. In that case, whenthe dummy load circuit 1001 receives information based on a switchingsignal with a duty ratio of 50% from the motor control board 302, thereis no fear that the pulse cannot be sensed. Thus, the reliability ofcontrol can be improved further.

Based on the aforementioned contents, the dummy load circuit 1001 canoutput a signal about carrier frequency information from the motorcontrol board 302 directly to the feedback output circuit. Even when thecontrol system of the inverter 301 is changed to a two-phase modulationsystem, a load similar to that of the gate drive circuit 106 can bereproduced in accordance with the carrier frequency so that thereliability of feedback control can be improved. Thus, the load of thefeedback output circuit may follow an actual load even when the carrierfrequency increases and the load of the output circuit of the powersupply circuit increases. Thus, the output voltage drop, that is, thegate voltage drop of each switching device can be suppressed when thecarrier frequency increases, so that it is possible to suppressdeterioration of power efficiency in the inverter. Further, the loadcurrent is outputted not by charging/discharging a capacitance byswitching but through DC resistances. Thus, the ripple noise isprevented from appearing on the output voltage of the feedback outputcircuit, so that stable control can be performed.

Fourth Embodiment

FIG. 12 represents a fourth embodiment of the invention, whosefundamental configuration is similar to that of the third embodiment.The motor control circuit 302 transmits three-phase PWM signals to adummy load circuit of a feedback output circuit 1203. For example, themotor control circuit 302 transmits a UN gate driving PWM signal 1204, aVN gate driving PWM signal 1205 and a WN gate driving PWM signal 1206 tothe dummy load circuit of the feedback output portion 1203.

The dummy load circuit 1201 has three dummy load circuits 201 shown inthe first embodiment. Three PWM signals can be inputted to the dummyload circuits 201 respectively. The dummy load circuits 201 share theload resistance 410 and the load capacitance 413 so as to simplify thecircuit configuration. In a more specific configuration, the externalinput signal buffer 411, the driver 412 and a load capacitance 1304 areprovided for the signal 1204. The load capacitance 1304 is 1/3 as highas the load capacitance 409 shown in FIG. 5 of the first embodiment. Inthe same manner, the signal level conversion circuit 411, the driver 412and a load capacitance 1305, 1306 are provided for each signal 1205,1206. The load capacitance 1305, 1306 is 1/3 as high as the loadcapacitance 409.

Based on the aforementioned contents, the dummy load circuit 1201 canoutput a signal about carrier frequency information from the motorcontrol board 302 directly to the feedback output circuit. Even when thecontrol system of the inverter 301 is changed to a two-phase modulationsystem, a load similar to that of the gate drive circuit 106 can bereproduced in accordance with the carrier frequency so that thereliability of feedback control can be improved. In addition, the loadresistance 410 and the load capacitance 413 are shared in the dummy loadcircuit 1201 so that the configuration of the dummy load circuit 1201can be simplified, and further the average load of the three phases U, Vand W can be reproduced. Thus, the ripple noise on the output voltage ofthe feedback output circuit 1203 can be relaxed.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A power conversion device comprising: an inverter circuit whichconverts DC power into AC power and which includes a plurality ofswitching devices constituting an upper arm and a lower arm; a controlcircuit which controls the plurality of switching devices; a drivecircuit which drives the plurality of switching devices based on asignal from the control circuit; and a power supply circuit whichsupplies power to the drive circuit; wherein: the control circuitcontrols a power supply voltage to be outputted from the power supplycircuit to the drive circuit; the drive circuit drives the plurality ofswitching devices based on a carrier frequency and the power supplyvoltage; the power supply circuit includes a transformer and a feedbackoutput circuit, the transformer including a primary coil to which avoltage is supplied from a battery, and a plurality of secondary coilsto which voltages are supplied through the primary coil respectively;among the secondary coils, a first secondary coil outputs a voltage tothe drive circuit; among the secondary coils, a second secondary coiloutputs a voltage to the feedback output circuit; and the feedbackoutput circuit includes a dummy load circuit which controls the voltageto be outputted to the primary coil based on a change of the carrierfrequency.
 2. A power conversion device according to claim 1, wherein:the inverter circuit includes U-phase, V-phase and W-phase circuitsbeing series circuits in each of which the upper arm and the lower armare connected in series; the drive circuit has a plurality of gate drivecircuits corresponding to the upper arm and the lower arm forming theU-phase, V-phase and W-phase circuits, respectively; the dummy loadcircuit includes a first switch unit, a second switch unit, a capacitorand a resistor; the first switch unit and the second switch unit areconnected in series to form a series circuit, the first switch unitbeing disposed on a higher potential side than the second switch unit;the capacitor is connected in parallel with the second switch unit; andthe resistor is connected in parallel with the series circuit.
 3. Apower conversion device according to claim 2, wherein: capacitance ofthe capacitor is substantially equal to capacitance of a capacitorconnected in parallel between a gate and an emitter of one of theswitching devices forming the inverter circuit.
 4. A power conversiondevice according to claim 2, wherein: a resistance value of the resistoris equal to a resistance value between a positive electrode connectionpoint of each gate drive IC belonging to the drive circuit and anegative electrode connection point of the gate drive IC.
 5. A powerconversion device according to claim 3, wherein: a resistance value ofthe resistor is equal to a resistance value between a positive electrodeconnection point of each gate drive IC belonging to the drive circuitand a negative electrode connection point of the gate drive IC.
 6. Apower conversion device according to claim 1, wherein: the secondsecondary coil has a smaller number of turns than the primary coil.
 7. Apower conversion device according to claim 2, wherein: the secondsecondary coil has a smaller number of turns than the primary coil.
 8. Apower conversion device according to claim 1, wherein: a ratio betweenthe number of turns of the primary coil and the number of turns of thesecond secondary coil is equal to a ratio between the voltage of thebattery and the voltage for driving the switching devices.
 9. A powerconversion device according to claim 2, wherein: a ratio between thenumber of turns of the primary coil and the number of turns of thesecond secondary coil is equal to a ratio between the voltage of thebattery and the voltage for driving the switching devices.
 10. A powerconversion device according to claim 1, wherein: the dummy load circuitincludes a plurality of switching circuits and a first resistor, each ofthe switching circuits including a resistor and a switching deviceconnected in series; the first resistor is connected in parallel withthe switching circuits; and the control circuit changes the number ofswitching circuits in which electric conduction should be secured, inaccordance with a change of the carrier frequency.
 11. A powerconversion device according to claim 10, wherein: the plurality ofswitching circuits include a first switching circuit, a second switchingcircuit and a third switching circuit; the control circuit secureselectric conduction in the first switching circuit when the carrierfrequency is not lower than a first predetermined value, secureselectric conduction in the first switching circuit and the secondswitching circuit when the carrier frequency is not lower than a secondpredetermined value, and secures electric conduction in the firstswitching circuit, the second switching circuit and the third switchingcircuit when the carrier frequency is not lower than a thirdpredetermined value; and the first predetermined value is smaller thanthe second predetermined value, and the second predetermined value issmaller than the third predetermined value.
 12. A power conversiondevice according to claim 2, wherein: the feedback output circuitincludes a plurality of dummy load circuits corresponding to the U-phasecircuit, the V-phase circuit and the W-phase circuit, respectively. 13.A power conversion device according to claim 12, wherein: information ofthe carrier frequency is outputted from the control circuit.